Adam.Fox@netapp.com gives a good description of what causes these files, in particular that it's all a matter for NFS clients, but I don't think it is really correct to say
[...] That's what causes .nfs files. It's part of the NFS spec to do so.
Not the "NFS spec" in the sense of RFCs 1094, 1813, or 3010, it isn't! (Unless I've overlooked something in them, which is certainly possible.)
It is rather a long-established "trick" (it goes right back to the original Sun implementations) used by NFS client implementations to simulate the behaviour of local Unix filing systems when an open file is unlinked.
Chris Thompson University of Cambridge Computing Service, Email: cet1@ucs.cam.ac.uk New Museums Site, Cambridge CB2 3QG, Phone: +44 1223 334715 United Kingdom.